In order to beat Screaming Mantis it is not about shooting her, but to shoot her dolls. The Mantis doll controls the living such as Meryl and any living Haven Troopers, while The Sorrow doll controls all the dead Haven Troopers. First, use the syringe to cut the ropes that causes you to miss. Second, take a weapon out with huge firepower and fast reload time. Now, shoot the Mantis doll before Meryl tries to kill you and blow her brains out as well. Continue to shoot until she drops the doll, and then pick it up. Optionally, after you gain one doll, you can go after the other one. When you are ready for the Screaming Beauty fight, use the newly acuqired Mantis doll to control her (equip as weapon, shoot, hold L1 and shake controller) until she dies. Either way you gain the Screaming Mantis doll.